A traditional seat belt device installed on a vehicle seat in an automobile or the like restrains an occupant using the seat belt in an emergency condition where deceleration acts on the vehicle, such as in a collision, (hereinafter referred to simply as in emergency). One typical example of the seat belt device of this type is a three-point seat belt device, which is widely known and frequently used. In such a typical known three-point seat belt device, a seat belt with its leading end fixed on a vehicle body withdrawn from a seat belt retractor is guided toward an occupant by a belt guide. A tongue is slidably supported on the guided seat belt. In this case, the seat belt penetrates through an elongated seat belt insertion hole in the tongue. The seat belt is attached to the occupant by engagement of the tongue into the buckle fixed on the vehicle body.
In a state where the seat belt is attached to the occupant, the seat belt between the belt guide and the tongue functions as a shoulder belt attached to a shoulder and chest of the occupant, and the seat belt between the end fixed on the vehicle body and the tongue functions as a lap belt attached to the lap (waist) of the occupant. In emergency, which is described above, the locking mechanism of the seat belt retractor is activated and impedes withdrawal of the seat belt, and thus the shoulder belt restrains the shoulder and chest of the occupant and the lap belt restrains the waist of the occupant.
In such a seat belt device, typically, even when the locking mechanism of the seat belt retractor is activated in emergency, the inertial force of the occupant causes the lap belt to slightly extend outward. This makes it difficult to effectively restrain the occupant by the seat belt.
A tongue and seat belt device capable of enabling the tongue to smoothly move relative to a seat belt in a normal operation of the tongue and of suppressing outward extension of a lap belt in emergency by locking the seat belt using a locking member provided to the tongue are proposed (see, for example, Patent Literature 1).
In the tongue described in Patent Literature 1, the metal locking member having an elongated shaft shape penetrates through guide holes in a metal tongue body in a longitudinal direction of the guide holes, and the opposite end portions of the locking member are urged by coil springs. In normal operation, the locking member is set in an unlocking position where it does not lock the seat belt. In emergency, the locking member is set in a locking position where it locks the seat belt.